In wire electric discharge machining apparatuses using water as a machining liquid, there are known wire electric discharge machining apparatuses in which iron-based materials such as die steel or tool steel corrode when the iron-based materials are machined.
In order to prevent corrosion of iron-based metals in water, it is useful to add a passivator to the water. Meanwhile, a machining liquid in a wire electric discharge machining apparatus needs to maintain a low conductivity (electric conductivity) of about 70 μS/cm or smaller. However, if a passivator is directly added to a machining liquid, it is very difficult to maintain a low conductivity, and thus it becomes difficult to perform stable discharge.
For this reason, Patent Literature 1 discloses a technology using an anion-exchange resin including a passivator supported thereon, as a method of adding a passivator in a state in which a low conductivity is maintained.
Specifically, as a resin composition that does not increase the conductivity of water, an anion-exchange resin, which includes nitrite ions (NO2−) serving as anticorrosive ions and one or more kinds of carbonate ions (CO32−), hydrogen carbonate ions (HCO3−) and hydroxide ions (OH−) bound thereto and serves as an anticorrosive resin, are used, and these anions are dissolved in water. Also, there is disclosed a technology for suppressing the water to a predetermined conductivity (about 10 μS/cm or less) by using that anticorrosive resin together with a water purifying resin composed of a hydrogen-ion (H+) form resin and a hydroxide-ion (OH−) form resin in a case of applying that anticorrosive resin to a metal cleaning and storing machine, and making a portion of the water flow into the water purifying resin if the conductivity of the water increases.
Patent Literature 2 discloses a technology for converting a machining liquid into an alkaline aqueous solution and suppressing corrosion of a metal material insusceptible to passivating, such as an ultra-hard material, by using an anticorrosive resin including anticorrosive ions supported thereon, and a water purifying resin composed of a H+ form resin and a OH− form resin together, and switching whether to make the machining liquid flow into each of the anticorrosive resin and the water purifying resin, according to a conductivity value.